As wireless local area networks (WLANs) are becoming ubiquitous, there is an increased demand in multimedia transmissions for both professional and personal uses. Multimedia applications are sensitive to delays and losses and require special Quality of Service (QoS) support in order to provide a satisfying user experience. The 802.11e standard addresses these needs by providing a framework for centralized scheduling and admission control algorithms, please see, e.g., WO2005/011307. The scheduling and admission control algorithms themselves are not standardized, but vendor dependent. Special scheduling algorithms for WLANs have been developed, which take into account the characteristics of WLANs, such as time-varying channels, multiple transmission rates, location-dependent errors as a result of the mobility of the users of the WLAN, and interference from neighboring WLANs.
The Air Fair Scheduling algorithm and its related admission control algorithm prevent streams which experience increased number of retransmissions or reduction in physical transmission rate (PHY-rate), due to worsening link quality, from affecting the QoS of streams whose link quality has not changed. For the Air Fair Scheduling algorithm, please see, e.g., the discussion of time-fairness and of TF-WFQ (time fair-weighted fair queuing) in WO2005/125124 (PHUS040231) or “Cross-Layer Wireless Multimedia Transmission: Challenges, Principles and New Paradigms”, M. van der Schaar and N. Sai Shankar, IEEE Wireless Communications, Vol. 12(4), Aug. 2005, pp 50-58. For the related admission control algorithm, please see, e.g., WO2005/011307.
The admission control algorithm, executed in the admission control unit (ACU), decides whether or not the scheduler can accommodate a stream based on, among other things, the stream's traffic characteristics (mean data rate and peak data rate, burst size, average packets size), the minimum PHY-rate, the maximum delay bound and the available resources of the scheduler. More specifically the ACU calculates the airtime quota needed by a particular stream in order to satisfy its QoS requirements.
An airtime policing module (APM) as presented in WO2006/077522, monitors and enforces the airtime allocated to outgoing streams of a station and thus completes the Air Fair Scheduling algorithm.
As specified in WO2006/077522, the airtime quota is policed at the granularity of a Service Interval (SI), such that the airtime used by a stream (referred to as an “admitted stream”) during each SI does not exceed the allocated transmission opportunity (TXOP-SI). The APM strictly enforces the TXOP-SI allocated for a particular stream.
WO2005/125124 relates to sharing a communications channel in wireless communications, and in particular to allocation of time to devices that communicate under a protocol such as 802.11e. The Time Fairness concept is utilized with an algorithm that specifies a way to distribute the time allocation in such a way that the delay requirements of all streams are not violated. Streams are prioritized based on highest ratio of, on the one hand, time remaining to be allocated to a stream in a service period and, on the other, the time remaining before the service period elapses.
U.S. Pat. No. 6,973,315 relates to a method and system for sharing over-allocated bandwidth between different classes of service in a wireless network. Traffic is transmitted for a first service class in excess of bandwidth allocated to the first service class, using unused bandwidth allocated to a second class. After transmitting traffic for the first service class in excess of bandwidth allocated to the first service class using unused bandwidth allocated to a second class, traffic for a third service class is transmitted in unused bandwidth remaining in the second service class.
In digital systems, the term “bandwidth” is being used to refer to “baud” i.e., the rate at which symbols may be transmitted through the system, or to refer to the channel capacity, i.e., the rate at which bits may be transmitted through the system.